The formation of ammonium nitrate can be avoided by starting from a metal oxide or metal carbonate which is dissolved in nitric acid. Although metal nitrate solutions which are free from ammonium nitrate are obtained in this way, this must be weighed against the fact that the starting materials are generally substantially more expensive and of lower purity than a metal obtained, for example, by electrolytic deposition or by decomposition of a metal carbonyl.
The metal to be dissolved is mixed with nitric acid. A stoichiometric excess of nitric acid on the one hand favors dissolution of the metal and on the other hand counteracts any possible hydrolysis of the metal nitrate already formed in the aqueous solution. For reasons of simple handling, the nitric acid should have a concentration of from 20 to 65% by weight. The metal can be treated with a correspondingly concentrated aqueous nitric acid, or a more concentrated nitric acid can be fed to the metal mixed with water. The nitric acid can be added continuously or in portions. While it is possible to add all the nitric acid necessary in one portion, it may in some cases be advantageous to spread the addition of the nitric acid over several portions to allow the reaction to proceed in a controlled manner.
The nitric acid is usually employed in excess relative to the metal to be dissolved. Since the dissolution of metal in piece form is very slow at room temperature, it is advisable to work at elevated temperatures. Nevertheless, the dissolution, in particular of manganese, chromium, nickel, and cobalt, requires a considerable time even at temperatures of 80.degree. C. and above. Although the dissolution can be simplified by using particularly finely divided metal, the metal to be processed on an industrial scale is usually not in powder form, but instead in piece form, so that this time-saving procedure cannot be used.
When finely divided metal is used, there is also the disadvantage of the formation, caused (for example) by stirring, of undesired metal dust. For this reason, the use of larger metal particles, such as microspheres, granules, electrode cuttings, brickettes, or crowns is preferred.
As a consequence of the increasing reactivity of nitric acid at higher temperatures, it should be insured that the reaction vessel is resistant to nitric acid. The material of the reaction vessel frequently sets limits to the use of elevated temperatures.
According to a process for preparation of nickel nitrate, to prevent the formation of ammonium nitrate, described in the Russian Journal of Inorganic Chemistry 1959, 11, 1122, it is recommended to add 30% hydrogen peroxide during the dissolution of metallic nickel in nitric acid. Based on ammonium nitrate, at least stoichiometric amounts of hydrogen peroxide are required. Since large amounts of hydrogen peroxide are additionally consumed under the reaction conditions due to decomposition, this process is also unacceptable economically in view of the difficulties in the handling of 30% strength hydrogen peroxide.
According to a procedure published in Dorofeeva et al., Khim. Prom-st. (Moscow) 1974, (8), 603-6, [Chem. Abstracts 81, 172286m (1974)], the formation of ammonium nitrate during the preparation of nickel nitrate is suppressed by adding metallic copper to the nickel to be dissolved or by using copper-containing nickel for the dissolution in nitric acid. It is disadvantage that the nickel nitrate solution obtained is contaminated with copper nitrate, as this material must be removed using separate purification steps.